1. Field of the Invention
This invention relates to an acceleration and deceleration control method executed by a controller (a numerical control device) which controls automatic machines such as various working machines, and more specifically, to an acceleration and deceleration control method suitably applied to automatic machines such as working machines available for high-speed positioning.
2. Description of the Prior Art
It is well known that an acceleration and deceleration control is performed along with positioning when performing servo control of an automatic machine such as a working machine with a numerical control device. For acceleration and deceleration control in positioning, a name suited to the pattern of a graph showing a time-speed transition is given, so that acceleration and deceleration control of a linear pattern, a bell pattern or the like is well known heretofore. For acceleration and deceleration control of a bell pattern, the left-side graph of FIG. 2 shows a time-speed transition, while the right-side graph acceleration) and speed. A restricted acceleration curve is also shown in the right-side graph.
The restricted acceleration curve illustrates an acceleration which can be outputted, determined based on machine friction, gravity, motor output torque performance and the like, as a function of speed. As shown in FIG. 2, actual acceleration (acceleration to be actually outputted) in acceleration and deceleration control varies within the interior range of the restricted acceleration curve, since the acceleration and deceleration control is performed within the foregoing range.
One of the features of the acceleration and deceleration control in the prior art as represented by the above control of the bell pattern is that, for the relation between speed and acceleration, the low speed part is symmetrical with respect to the high-speed part. That is, a curve representing the relation between speed and actual acceleration is trapezoidal in shape. On the other hand, in a machine such as a working machine having axes subjected to acceleration and deceleration control, it is usual that, for an acceleration and deceleration performance which can be outputted, the low speed part is not symmetrical with the high speed part due to friction and gravity acting on the machine parts or due to the torque characteristics of a motor which drives an axis.
Accordingly, a restricted acceleration curve, which is a function of a speed and represents an acceleration and deceleration performance which can be outputted, is generally provided in the shape of asymmetry with respect to speed. A typical restricted acceleration curve (which is shown in FIG. 2 along with the acceleration-speed curve) is shown in FIG. 5 with a horizontal axis denoted as speed and a vertical axis as acceleration.
As is apparent from the shape of the curve in FIG. 5, an acceleration and deceleration performance which can be outputted is restricted particularly in the high-speed part. For this reason, in the prior art, there is provided a method for realizing sufficient acceleration and deceleration control as much as possible within the range of the restricted acceleration curve by generating an actual acceleration curve a part of which comes into contact with the restricted acceleration curve, even though, for the actual acceleration curve, the low speed part is symmetrical with respect to the high speed part (See Japanese Patent Application Laid-open No. 4-362710, for instance).
However, as the acceleration and deceleration performance according to the above method is also restricted by the high-speed part of the restricted acceleration curve, full acceleration meeting the performance cannot be realized in the low-speed part. In other words, a most part of the acceleration performance is not effectively utilized in the low speed part, with the result that it takes extra time for positioning and acceleration and deceleration before and after the positioning.